The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 100 46 0550.0 filed on Sep. 18, 2000, the diclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The present invention relates to a roll, in particular a roll for the smoothing of paper webs. The roll has a hard roll core made of, in particular, metal and is provided on its outside with an elastic covering layer which includes an elastic matrix material and fibers embedded into the matrix material. The invention is further directed to a method for the manufacture of such a roll.
2. Discussion of Background Information
Elastic rolls of this kind are used, for example, in the satining of paper webs. Here, one elastic roll forms, in each case together with a hard roll, a press gap through which the paper web to be treated is guided. The hard roll has a very smooth surface consisting, for example, of steel or chilled cast iron and is responsible for the smoothing of that side of the paper web facing it. The elastic roll acting on the opposite side of the paper web effects a homogenizing and compacting of the paper web in the nip. The order of magnitude of the rolls ranges from lengths of 3 m to 12 m and diameters from 450 to 1500 mm. They can withstand line forces of up to 600 N/mm and compressive stresses of up to 130 N/mm2.
To achieve a smoothing on both sides of the paper web, normally a plurality of roll pairs of this kind are successively disposed in a calender, with each of the two sides of the paper web alternately coming into contact now with the hard metal roll and now with the elastic roll in successive gaps. Since the surface of the elastic roll has a relatively high roughness with respect to the extremely smooth surface of the hard roll, the previously achieved smoothing result is at least partly again ruined in each case at the side of the paper web which is being guided over the elastic roll in the current smoothing gap.
A further problem lies in the fact that the required multi-roll calenders are expensive and the transport speed of the paper web is limited when multi-roll calenders are used. This is particularly disadvantageous since the trend in paper manufacturing is towards carrying out satining in an online operation. The paper web exiting the paper making machine or coating machine is here guided directly through the paper smoothing apparatus (e.g., a calender), whereby higher demands than previously are made on the rolls of the smoothing apparatus, particularly with respect to temperature resistance. As a result of the high transportation speeds of the paper web required in online operation and the high rotation speeds of the calender rolls associated with this, their nip frequency, that is the frequency with which the covering is compressed and relieved of its load again, is increased, which in turn leads to increased roll temperatures. These high temperatures arising in online operation result in problems which can even result in the destruction of the plastic coatings in known elastic rolls. On the one hand, with known plastic coatings, maximum temperature differences of around 20xc2x0 C. are permissible over the width of the roll and, on the other hand, the plastics conventionally used for the coating have a substantially higher coefficient of thermal expansion than the conventionally used steel rolls or chilled cast-iron rolls so that high axial stresses occur between the steel roll or the chilled cast-iron roll and the plastic coating connected to it due to an increase in temperature.
So-called hot spots, at which a peeling or even a breaking open of the plastic layer occurs, can arise due to these high stresses in conjunction with hot regions occurring particularly in spot form.
These hot spots occur in particular when, in addition to the mechanical stresses and the relatively high temperature, crystallization spots are present in the form of, for example, defective adhesive bonds, deposits or above-average recesses in the elastic coating, for example due to creases or foreign bodies in the paper web. In these cases, the temperature at the crystallization spots can increase from the normal 80xc2x0 C. to 90xc2x0 C. to more than 150xc2x0 C., whereby the above-mentioned destruction of the plastic layer occurs.
To control the properties of the elastic covering layer, fillers are normally introduced into the matrix material in the form of fibers or powder. Depending on the quantity and physical property of these fillers, the physical properties of the elastic covering layer are dominated or influenced by the fillers. For example, the thermal conductivity of the elastic covering layer can be improved by using fillers having a high thermal conductivity.
The smoothness of the surface of the covering layer is normally achieved by an appropriate grinding and polishing of the covering layer. Due to the size of the normally used fillers, however, only a listed smoothness of the surface of the covering layer can be achieved. For example, fibers previously used as a filler typically have diameters between 8 xcexcm up to 20 xcexcm. Since these fillers come to lie at the surface in the grinding of the surface, and exit this in part, the smoothness of the surface of known elastic rolls is substantially lower than the smoothness of the known hard rolls.
The present invention provides for an elastic roll of the kind initially mentioned, and a method for the manufacture of such a roll, with which the result in the smoothing procedure is further improved with respect to conventional elastic rolls and the risk of the occurrence of hot spots is reduced.
Starting from a roll of the kind initially mentioned, the invention provides that the diameter of the fibers is less than 800 nm so that the surface of the elastic covering layer has an extremely high smoothness, in particular an Ra value of less than around 0.6 xcexcm, in that the thickness of the elastic covering layer amounts to between 3 and 20 mm and in that, in addition to the fibers, powdery fillers are embedded in the matrix material whose outer dimensions are in each case less than 1 xcexcm at least in one direction.
A corresponding method is characterized in that, to produce an extremely high smoothness of the elastic covering layer, in particular an Ra value of less than around 0.6 xcexcm, substantially only fibers are introduced into the elastic matrix material whose diameters are less than 800 nm, in that, in addition to the fibers, powdery fillers are introduced into the matrix material whose outer dimensions are in each case less than 1 xcexcm at least in one direction and in that the elastic covering layer is formed with a thickness of between 3 and 20 mm.
In a roll of the invention, exclusively fibers and powdery fillers (both generally termed fillers in the following) having dimensions which are many times smaller than the dimensions of conventional fillers are thus used. A plurality of advantages are achieved thereby. On the one hand, the surface of the elastic covering layer including these extremely small fillers is substantially smoother than the surface of conventional elastic rolls after the grinding and polishing since the filler sections projecting out of the surface have correspondingly small dimensions.
On the other hand, due to the small dimensions of the fillers, a substantially finer distribution of the fillers within the covering layer is possible, whereby both a better thermal conductivity and a higher strength of the covering layer is achieved. The improved thermal conductivity results in the high temperatures occurring in operation, in particular at defective locations, being dissipated very quickly so that the occurrence of hot spots is largely prevented. The higher strength which is achieved by the better homogeneity of the covering layer material also here results in a reduction in the probability of hot spots occurring.
The improved thermal conductivity is reached in particular in that a higher packing density of the fillers can be reached due to the reduced dimensions of the fillers. With this increased packing density, the quantity of the matrix material present between the fillers and normally having a low thermal conductivity is reduced so that the overall thermal conductivity of the elastic covering layer is improved. The powdery fillers arranged in these free matrix regions formed between the fibers furthermore have the effect that these regions also have an increased thermal conductivity, whereby the extremely high smoothness of the covering layer is maintained due to the selected dimensions of the powdery fillers.
Furthermore, the number of the required smoothing gaps can be reduced due to the improved surface smoothness since a high smoothness of the paper web can also be achieved at the side of the paper web associated with the elastic roll and the smoothing result previously achieved by the hard roll is not again degraded by the elastic roll as with the known rolls.
In accordance with a further advantageous embodiment of the invention, the diameter of the fibers is less than around 500 nm, in particular less than around 200 nm. The smaller the diameters of the fibers are selected, the smoother the surface of the covering layer is and, associated therewith, the better the smoothing result is. This also applies with respect to the powdery fillers so that the outer dimensions of the powdery fillers are preferably less than 800 nm at least in one direction, in particular less than around 500 nm, preferably less than around 200 nm.
The fibers are preferably formed as carbon fibers, aramide fibers or glass fibers. A mixture of these fibers is also possible. The powdery fillers are preferably made of the same material as the fibers, but can basically also consist of different materials with a high thermal conductivity. The fibers can advantageously be mutually connected, for example twisted or knitted, and advantageously be present in the form of fiber rovings or of a fiber fleece. An improved thermal conductivity with respect to conventional covering layers is achieved due to the connection of the fibers with a length, for example, of around 10 mm or also less in conjunction with the increased pacing density.
Predominantly the diameter of the fibers is decisive for the desired surface smoothness when one of the ends of the largest proportion of the fibers projects from the surface of the covering layer during grinding. The smaller the diameter of the fibers, the higher the surface smoothness of the covering layer thus is after grinding and polishing. If the fibers predominantly lie at the surface of the covering layer with their longitudinal sides, then the surface smoothness is likewise improved with respect to conventional covering layers due to the reduced thickness, even if the length of the fibers is selected as normally. The use of carbon fibers is advantageous in that these have a good thermal conductivity. A fast heat dissipation via the carbon fibers is ensured in this manner when the roll heats up in operation.
The same applies in analogous manner for the powdery fillers. In particular with a formation as essentially round or spherical particles, their diameter is less than 1 xcexcm.
The fillers can preferably be uniformly distributed in the matrix material, with a very homogeneous mixture and, associated therewith, a very high strength of the covering layer being achieved due to the low size of the fillers with a simultaneously very good thermal conductivity. The improved thermal conductivity is in particular achieved in that the material of the fillers is selected such that it has a higher thermal conductivity than the matrix material.
In accordance with a further advantageous embodiment of the invention, the surface of the elastic covering layer has an Ra value of less than around 0.5 xcexcm, in particular of less than around 0.2 xcexcm, preferably of less than around 0.1 xcexcm. The elastic roll thus has a surface smoothness which lies in the order of magnitude of the surface smoothness of the hard roll so that on running through a smoothing gap both sides of the paper web are essentially smoothed with the same quality. Depending on the desired result, some or almost all of the smoothing gaps of a multi-roll calender can thus be omitted so that in the ideal case a sufficient smoothing result is achieved with a single smoothing gap.
The manufacture of the elastic roll can take place in a known manner, for example, by an injection, casting or winding method onto the roll core, withxe2x80x94in accordance with the inventionxe2x80x94extremely small powdery fillers and extremely thin fibers being introduced into the elastic matrix material or being coated therewith in a winding method. The fibers can preferably be present in the form of fiber rovings so that extremely thin layers of fiber rovings can be wound onto the roll core in accordance with the extremely thin diameter of the fibers. The rovings are preferably impregnated with the matrix material before or during the winding or treated with the matrix material after the winding onto the roll core. The use of a fiber fleece which ensures a good thermal conductivity and which can be impregnated with the matrix material in the same way is also advantageous.
The invention provides for a roll for smoothing a web comprising a hard roll core. An elastic covering layer is arranged on an outside surface of the hard roll core. The elastic covering layer comprises an elastic matrix material and fibers embedded in the elastic matrix material. At least some of the fibers comprise a diameter which is less than 800 nm. A surface of the elastic covering layer has an extremely high smoothness.
The extremely high smoothness may be defined by an Ra value of less than around 0.6 xcexcm. The elastic covering layer may comprise a thickness of between 3 and 20 mm. The roll may further comprise powdery fillers embedded in the elastic matrix material. At least some of the powdery fillers may comprise an outer dimension which is less than 1 xcexcm at least in one direction. The web may comprise a paper web. The hard roll core may comprise a metal. The metal may comprise one of steel and chilled cast iron. The diameter may be less than around 500 nm. Alternatively, the diameter may also be less than around 200 nm. The roll may further comprise powdery fillers embedded in the elastic matrix material, wherein at least one outer dimension of the powdery fillers is smaller than around 1 xcexcm. At least one outer dimension of the powdery fillers may be smaller than around 800 nm or than around 500 nm. At least one outer dimension of the powdery fillers may be smaller than around 200 nm. The powdery fillers may be smaller than 1 xcexcm. The powdery fillers may be smaller than around 800 nm. The powdery fillers may also be smaller than around 500 nm. Furthermore, the powdery fillers may be smaller than around 200 nm. At least some of the powdery fillers may comprise one of substantially round particles and spherical particles. At least some of the powdery fillers may comprise carbon. At least some of the fibers may comprise carbon fibers. The powdery fillers may comprise a material that has a higher thermal conductivity than that of the elastic matrix material. The fibers may comprise a material that has a higher thermal conductivity than that of the elastic matrix material. The powdery fillers may be uniformly distributed in the elastic matrix material.
The fibers may be uniformly distributed in the elastic matrix material. The surface of the elastic covering layer may have an Ra value of less than around 0.5 xcexcm. The surface of the elastic covering layer may also have an Ra value of less than around 0.2 xcexcm. Furthermore, the surface of the elastic covering layer may have an Ra value of less than around 0.1 xcexcm.
The invention also provides for a method of making a roll for smoothing a web which includes a hard roll core, an elastic covering layer arranged on an outside surface of the hard roll core, the elastic covering layer comprising an elastic matrix material and fibers embedded in the elastic matrix material, and at least some of the fibers comprising a diameter which is less than 800 nm, the method comprising introducing the fibers into the elastic matrix material to form the elastic covering layer, and coupling the elastic covering layer with the hard roll core, wherein a surface of the elastic covering layer has an extremely high smoothness.
The method may further comprise forming the surface of the elastic covering layer with an Ra value of less than around 0.6 xcexcm. The method may further comprise introducing powdery fillers into the elastic matrix material. At least some of the powdery fillers may comprise at least one outer dimension which is smaller than 1 xcexcm. The method may further comprise forming the elastic covering layer with a thickness of between 3 and 20 mm. The method may further comprise forming the elastic covering layer on the hard roll core by one of an injection method, a casting method and a winding method. The method may further comprise forming the elastic covering layer on the hard roll core by one of an injection method, a casting method and a winding method. The method may further comprise applying the fibers and the elastic matrix material onto the hard roll core. The method may further comprise applying the fibers, the powdery fillers, and the elastic matrix material onto the hard roll core.
The invention also provides for a roll for smoothing a web comprising a hard roll core. An elastic covering layer is arranged on an outside surface of the hard roll core. The elastic covering layer comprises an elastic matrix material and each of fibers and fillers embedded in the elastic matrix material. At least some of the fibers comprise a diameter which is less than 800 nm. At least some of the fillers comprise at least one outer dimension which is less than 1 xcexcm. A surface of the elastic covering layer has an Ra value of less than around 0.6 xcexcm.